DESCRIPTION

At return, the integer running_handles points to will contain the number
of still running easy handles within the multi handle. When this number
reaches zero, all transfers are complete/done. Note that when you call
curl_multi_socket_action(3) on a specific socket and the counter
decreases by one, it DOES NOT necessarily mean that this exact socket/transfer
is the one that completed. Use curl_multi_info_read(3) to figure out
which easy handle that completed.

The curl_multi_socket_action(3) functions inform the application about
updates in the socket (file descriptor) status by doing none, one, or multiple
calls to the socket callback function set with the CURLMOPT_SOCKETFUNCTION
option to curl_multi_setopt(3). They update the status with changes
since the previous time the callback was called.

Get the timeout time by setting the CURLMOPT_TIMERFUNCTION option with
curl_multi_setopt(3). Your application will then get called with
information on how long to wait for socket actions at most before doing the
timeout action: call the curl_multi_socket_action(3) function with the
sockfd argument set to CURL_SOCKET_TIMEOUT. You can also use the
curl_multi_timeout(3) function to poll the value at any given time, but
for an event-based system using the callback is far better than relying on
polling the timeout value.

The easy argument is a pointer to the easy handle that deals with this
particular socket. Note that a single handle may work with several sockets
simultaneously.

The s argument is the actual socket value as you use it within your
system.

The action argument to the callback has one of five values:

CURL_POLL_NONE (0)

register, not interested in readiness (yet)

CURL_POLL_IN (1)

register, interested in read readiness

CURL_POLL_OUT (2)

register, interested in write readiness

CURL_POLL_INOUT (3)

register, interested in both read and write readiness

CURL_POLL_REMOVE (4)

unregister

The socketp argument is a private pointer you have previously set with
curl_multi_assign(3) to be associated with the s socket. If no
pointer has been set, socketp will be NULL. This argument is of course a
service to applications that want to keep certain data or structs that are
strictly associated to the given socket.

The userp argument is a private pointer you have previously set with
curl_multi_setopt(3) and the CURLMOPT_SOCKETDATA option.

RETURN VALUE

CURLMcode type, general libcurl multi interface error code.

Legacy: If you receive CURLM_CALL_MULTI_PERFORM, this basically means
that you should call curl_multi_socket(3) again, before you wait for
more actions on libcurl's sockets. You don't have to do it immediately, but
the return code means that libcurl may have more data available to return or
that there may be more data to send off before it is "satisfied".

In modern libcurls, CURLM_CALL_MULTI_PERFORM or
CURLM_CALL_MULTI_SOKCET should not be returned and no application needs
to care about them.

NOTE that the return code is for the whole multi stack. Problems still might have
occurred on individual transfers even when one of these functions
return OK.

TYPICAL USAGE

1. Create a multi handle

2. Set the socket callback with CURLMOPT_SOCKETFUNCTION

3. Set the timeout callback with CURLMOPT_TIMERFUNCTION, to get to know what
timeout value to use when waiting for socket activities.

4. Add easy handles with curl_multi_add_handle()

5. Provide some means to manage the sockets libcurl is using, so you can check
them for activity. This can be done through your application code, or by way
of an external library such as libevent or glib.

6. Wait for activity on any of libcurl's sockets, use the timeout value your
callback has been told

7, When activity is detected, call curl_multi_socket_action() for the
socket(s) that got action. If no activity is detected and the timeout expires,
call curl_multi_socket_action(3) with CURL_SOCKET_TIMEOUT

8. Go back to step 6.

AVAILABILITY

This function was added in libcurl 7.15.4, and is deemed stable since
7.16.0.